Chair of Solid State Physics


Research report 1997

Figure 1. Evolution of self-organized InAs islands as a function of coverage with GaAs. The AFM image shows the topology of uncapped InAs islands in the lower right. Upon capping with 40Å GaAs (lower left) the morphology drastically changes, the dots evolve into volcano-like structures. Further increase of the GaAs cap layer thickness will fill the voids in the center of the volcanoes.


Modern semiconductor technology nowadays combines more than ten million different transistors on a single chip barely as big as a thumbnail to form an extraordinary complex and sophisticated circuit. Following Moore's law, this very large scale integration will proceed over roughly the next ten years until a single element on a chip will be scaled down to about a size of 100 nm. This typical dimension of a single device, however, represents a barrier, beyond which the basic operation of an electronic device starts to be based on fundamentally different mechanisms as compared to the conventional ones.

In a classical silicon MOSFET, for example, the principle of operation is based upon the statistical motion of about 10'000 electrons per square micron, whose number may be varied by an external electrode via electric fields. This movement takes place close to the relatively rough silicon/silicon dioxide interface and is described by diffusive processes, similar to the Brown's motion of molecules.

If, however, the dimension of a device becomes comparable or even smaller than the typical distance between two scattering events, the electrons start to move ballistically, like the balls on a billiard table. Moreover, at these small sizes, the number of electrons within a single device starts to approach one. For even smaller devices, their size becomes comparable to the wavelength of the electrons themselves - typically some ten nanometers in this case: The description of the electrons behaving like little charged spheres starts to fail and to require for a quantum mechanical formulation of the device.

In our group, we investigate the electronic and optical properties of specially tailored semiconductor structures with typical dimensions of the order or less than 100 nm. Our goal is the detailed understanding of the new physical phenomena associated with a dramatic reduction of size, to explore new grounds for future device applications, and to be prepared for the day when nano-electronics will take over the role of micro-electronics to boldly go where no person has ever gone before!

The research in our group is based on three fundamental prerequisites :

  • Nanotechnology
  • Sophisticated electronic and optical experimental techniques
  • Quantum mechanical concepts and analysis

Starting from suited semiconductor layered systems, we first have to prepare the desired structures with lateral nanometer size dimensions. We use and develop different nanotechnologies that enable us to scale down the size of our structures to the size of the electronic wavelength. For this purpose, our nanotechnology labs are located in a dust free cleanroom area containing modern semiconductor processing equipment.

As we're always trying to be internationally competitive, we set up a large number of international co-operations with partners being specialized in the epitaxial growth of our high quality starting material. Meanwhile, our nanotechnological techniques are also transferred to different disciplines of leading edge research resulting in newly developed collaborations with highly qualified specialists in x-ray analysis, polymer physics and biophysics.

Secondly, we constantly develop and apply sensitive experimental techniques which enable us to chararcterize and to investigate the electronic and optical properties of our nanometer scale samples over the whole spectral range starting from DC over the microwave and infrared regime, the visible spectrum up to UV. At the same time, we are equipped with facilities allowing fo extremely low temperatures and high magnetic fields - invaluable tools for the detailed undersanding of the quantum mechanic phenomena in our devices.

A third prerequisite for our research is a detailed and fundamental theoretical analysis and understanding of nanophysics. Together with many theoretical groups and in a very fruitful atmosphere of collaboration, we try to develop new theories and techniques helping us to understand or to predict the many fascinating effects that we constantly facing. This is in particular important, as we are not studying systems already existing in nature but try to artificially tailor small pieces of this nature to behave in a desired fashion.

A. Transport Properties

Figure 2. Color-coded magnetocapacitance signal of an array of self-assembled quantum dots. Clearly visible are the two lowest lying (s-shell) states with a weak diamagnetic shift. The four peaks at intermediate energies constitute the p-shell which exhibits an orbital Zeeman splitting when a magnetic field is applied. Additional finestructure can be observed which replicates the basic features of the main spectrum.

A.1. Interaction of Surface Acoustic Waves (SAW) and Low-Dimensional Electron Systems

Figure 3. Hybrid-based voltage - controlled oscillator operating at room temperature. The device consists of a thin GaAs/InGaAs semiconductor quantum layer containing a quantum well with a two-dimensional electron system that is transferred onto a strongly piezoelectric substrate. On this substrate, a surface wave is propagating. The interaction between the wave and the electron system leads to a large, tunable change of the sound velocity and hence the operating frequency of the device.

Surface acoustic waves are modes of elastic energy which can propagate on the surface of different materials. If the substrate is piezoelectric, those waves are accompanied by electric fields which then propagate at the speed of sound. The electric fields of the wave can couple to the mobile carriers within a semiconductor structure and modify its electronic and elastic properties. By measuring the attenuation of the wave and the renormalization of the sound velocity we can, for instance, extract information on the dynamic conductivity of the electron system. We also investigate the possibility to use a SAW for a dynamical lateral potential modulation and we investigate the influence of a SAW onto the optical properties of an electron system. Our experiments presently cover the frequency range between 100 MHz and 6 GHz, corresponding to surface acoustic wavelengths between 30um and 500 nm, respectively.

The SAW - related research in 1997 was governed by three different two different topics:

  • Frequency dependent interaction between SAW and low-dimensional electron systems
  • Hybridization of piezoelectrics and semiconductors, exploring possible device applications
  • Optical investigations of the SAW - semiconductor interaction

Our recent progress in fabricating high frequency and multi-frequency SAW transducers enables us to expand our SAW transport studies well into the GHz regime. The possibility to generate multiple frequencies on a single device allows for a detailed investigation of the SAW - 2DES interaction as a function of the frequency. On standard semiconductor heterojunctions, however, the strength of this interaction is rather weak. For this reason, we use hybrid structures consisting of a strong piezoelectric (LiNbO3) the active semiconductor layer structure.

Two routes have been followed:

  • A sandwich-technique, where the semiconductor is mechanically pressed onto a strong piezoelectric, on which the SAW is propagating . Detailed experimental and theoretical investigations on the frequency dependence of the interaction in such sandwich structures have been carried out.
  • Quasi-monolithic hybridization. Here, a thin layer of the semiconductor structure containing the active heterojunction is removed from its substrate and tranferred onto a strongly piezoelectric host substrate.

    The SAW - 2DES interaction can be enlarged by two orders of magnitude as compared to the monolithic case. This interaction is now strong enough to become technologically very attractive. Room temperature operation is also possible. Together with the Siemens research lab, we are presently developping new concepts for a technological exploitation of this approach.

  • The last topic is the investiagtion of the influence of a SAW on the optical properties of a semiconductor quantum well. Here, we opened a completely new and exciting area of research and attracted a tremendous national and international interest. We observe a strong influence of the SAW on the optical properties of a quantum well . The photoluminescence of the quantum well can be completely quenched under the influence of the SAW. Moreover, we show that the SAW can act as a "Photon Conveyor Belt", where optical signals can be stored by the SAW in the semiconductor and may be re-assembled into light after very long delay times and at a remote location of the sample!

Further, and related studies, namely the influence of SAW on the optical properties of semiconductor quantum structures aim towards the use of SAW for the generation of lateral Bragg gratings by a modulation of the local refractive index of the system. Such lateral, tunable grating structures will be of severe importance for future acousto-opto-electric devices like modulators, switches and multiplexers. This work is performed under the sponsorship of the Bayerische Forschungsverbund FOROPTO

A.2. High-Resolution Capacitance Spectroscopy on Small Arrays of Self-Organized InAs Quantum Dots

To study small arrays of self-organized InAs quantum dots we have developed a high-resolution capacitance bridge working at cryogenic temperatures and in high magnetic fields. The noise level of the capacitance signal of the sample is reduced by means of an on-chip transistor. This transistor is realized by a HEMT located just a few mm´s away from the sample. With this set-up we can resolve the tunneling of single electrons. We employ this high-resolution technique to study the dependence of the magneto-capacitance spectra on the size of the dot ensemble. On a sample containing about 103 dots we were able to resolve the few-electron ground states for electron numbers ranging from zero up to 8. The obtained experimental energy level diagram shows the shell structure of zero-dimensional electron systems. For samples containing about 102 dots the capacitance spectra get more complicated. An additional fine structure is observed reflecting an ensemble property of the dots. In the limit of only 10-20 quantum dots sharp charging maxima corresponding to the tunneling of only a few electrons are observed. To study small arrays of self-organized quantum dots nm-size gate areas have to be defined (see figure). For this purpose we have employed an air-bridge technique.

Figure 4. AFM-picture of a sample for high-resolution capacitance spectroscopy. To reduce the effect of spurious capacitances, an air bridge technique has been employed. The active gate area covers only about ten self-assembled quantum dots!

A.3. Electron Transport in Antidot-Lattices

"Antidot" lattices can be considered the complementary structure to quantum dot lattices: While in the latter, an array of isolated islands of electrons is created, in the former an array of small voids is cut out of a two-dimensional electron gas. In the past, a number of novel effects have been observed both in classical as well as quantum transport, which result from the interplay between different length scales in antidot-lattices (lattice period, magnetic length, Fermi-wavelength). Whereas commonly circular voids are patterned, we have created lattices of cross-shaped antidots. Such lateral superlattices can equally be regarded as antidot arrays, as two-dimensional arrays of quantum point contacts or as arrays of coupled quantum dots. They are therefore promising candidates to study the the transition between different types of quantization, e.g., between the quantized conduction in 1-dimensional channels at zero magnetic field and the quantum Hall effect in 2-dimensional electron gases at high fields. In the antidot picture, the complex unit cell is expected to lead to more complex carrier dynamics, compared to the well-investigated case of simple (round) antidots. Indeed, a number of novel structures in the magnetotransport properties of "+"-shaped antidots have been observed. Puzzling at first, the origin of these feature could be clarified by a detailed analysis of the quasi-classical ballistic electron trajectories, which was performed in collaboration with the Regensburg University.

A.4. Ballistic Rectifier

Furthermore, antidot lattices have been investigated where the triangular shape of the basis breaks the left-right symmetry of the electron system. In low frequency transport these samples exhibit maxima in the magnetotransport at a magnetic field where the cyclotron diameter equals half the lattice period. This is caused by so-called skipping orbits and shows that it is indeed possible to transfer triangular shapes with straight edges into the two-dimensional electron gas. In high-frequency experiments we observe lateral photo-voltages which closely reflect the features in low-frequency transport caused by the presence of the antidots. This might be due to the fact that the broken symmetry of the antidot-lattice leads to a rectification of the high-frequency radiation.

To further investigate the influence of symmetry-breaking on transport properties in mesoscopic semiconductor structures, we have fabricated cross junctions with an embedded triangular antidot. As seen in the figure, a simple picture of ballistic electron transport predicts unusual transport behaviour in such a device: Independent of current direction, electrons ejected out of the narrow channels (S and D) will be reflected from the edges of the antidot towards the lower (L) channel. This way, a voltage is induced between the L and the U channel which does NOT change sign, when the input current is reversed. Indeed, a clear voltage was observed experimentally on such samples, which exhibits the above mentioned unusual symmetry relations.

A.5. Coulomb Blockade Phenomena in Quantum Dots

If very small islands of electrons are isolated from a two-dimensional electron gas via tunnel barriers, the charging energy needed to add additional electrons to the island can be higher than the thermal energy available. In such a case, which usually requires very low temperatures, electron transport through such a quantum dot is blocked.This so-called Coulomb blockade can be raised by tuning the islands energy with an additional plunger gate (see figure). A plunger gate sweep results in quasi-periodic conductance oscillations.

The difference between adjacent conductance maxima is the addition energy. In the simple constant interaction model this energy can be expressed as the sum of a classical capacitive charging term and a quantum-mechanical single particle energy level spacing.

We have performed Coulomb blockade measurements on a semiconductor quantum dot fabricated in a GaAs-AlGaAs-heterostructure and investigated the fluctuations and the distribution of the conductance peak spacings (9-97) obtained from these measurements. The statistical properties were compared to the predictions of random matrix theory (RMT) as this theory provides a very good description of the spectra of many complex systems.

It was found that the fluctuations in the peak spacings are larger than expected from RMT and the distribution of the spacings resembles a Gaussian rather than a Wigner-like probability distribution. This indicates that the peak spacings are strongly affected by the electronic interactions on the dot. However, both constant interaction model and RMT deal with single particle spectra. Until now, there is still put considerable theoretical effort on the understanding of the peak spacing distributions.

Figure 5. REM micrograph of an electrostatically defined quantum dot. Two tunnel contacts provide access to the transport properties of the dot, which can be controlled by using a central plunger gate. The lithograpically defined dot area is about 0.5 * 0.5 microns.

B. Intra- and Interband Spectroscopy

Figure 6. Interband absorption spectra of an array of self-assembled quantum dots. With increasing carrier density in the dots  the energetically low-lying transitions become 'Pauli-blocked' as the final states for the transitions are already occupied by electrons. Note the extreme sensitivity of the experiment as indicated by the very small absorption measured!

B.1. Electron Systems in Band Gap Engineered Quantum Systems

Modern crystal growth techniques like molecular beam epitaxy offer the unique advantage of tailoring the band edges of different semiconductor systems in a very controlled manner. A prominent example is the so-called parabolic quantum well (PQW), but also more complex systems can be realized.

In close co-operation with the research group of A.C. Gossard in Santa Barbara we concentrate on the investigation of the collective reponse of low-dimensional electron systems in such man-made semiconductor structures.

In 1997, we focused on the investigations of the properties in coupled quantum well structures on the basis of PQW.

We developed a technique, where the active layers of such a PQW structure can be selectively removed from their natural substrate and then be transferred onto another host substrate. There, laterally patterned gate electrodes can be used to selectively make contact to the two parts of the double layer, being separated from eachother by only some ten nanometers. Detailed investigations on the strength of the coupling between two (or more) high mobility electron systems have been carried out. Both DC transport as well as FIR spectroscopic techniques have been employed.

B.2.Intersubband Spectroscopy of InAs/AlSb Quantum Wells

The relatively small energy gap of some semiconductors like InAs and the related pronounced coupling of neighbouring bands leads to strong non-parabolic effects in the energy vs. momentum dispersion relation. Such a non-parabolicity reflects, e.g., in a strongly energy dependent effective mass. To study such effects we concentrate on a relatively new material combination, namely InAs/AlSb which also provides the deepest quantum wells available to date.

A number of experiments have demonstrated that the depolarisation field plays a very important role in the intersubband resonance of InAs quantum wells.

We have been investigating intersubband resonance in InAs quantum wells (provided by Herb Kroemer's group at UC Santa Barbara) for sometime yet there are still surprises! Experiments in 1997 showed how the linewidth is remarkably temperature independent from 4 K right up to 700 K. Other parameters, such as the transport mobility, vary tremendously in this temperature range. We have focused our attention on the scattering mechanisms which determine the intersubband resonance linewidth by putting together the information from experiments versus temperature and well width. A simple picture emerges. The collective nature of the intersubband resonance is crucial. Scattering is most effective when the intersubband resonance, or plasmon, can be scattered into degenerate single-particle excitations (Landau damping). This explains why the temperature dependence is weak (a very limited number of phonons can scatter the plasmon along its dispersion curve). The main scattering comes from well width fluctuations as they can scatter the plasmon into a damped region of the dispersion curve.

As many of our intersubband resonance absorption experiments are performed using a grating coupler technique, we investigated both experimenatlly as well as theoretically the influence of the grating design on the absorption of intersubband resonances in quantum well systems. It turns out that the design of the grating coupler strongly influences the lineshape and absorption strength of the observed resonances.

B.3. Photoconductive Response of Ins/AlSb Quantum Wells

Not only from the physics point of view this material combination is a very attractive candidate for semiconductor research. The unique band structure together with high carrier concentrations and small effective masses makes it very interesting for possible device applications. Here, we focus on the photonic properties and the optoelectronic response. Possible applications include infrared detectors, modulators, and switches in the infrared regime. In close co- operation with the research group of H. Kroemer at UC Santa Barbara (USA) we design and develop suitable layer structures to follow this route. In 1997, we could sucessfully demonstrate that an in-plane photoconductive signal of the InAs/AlSb structures can be used for both intersubband as well as interband spectroscopic detection. Studies of this photoconductivity in high magnetic fields provided detailed insight into the responsible relaxation processes.

B.4. Spectroscopy of Self-Organized InAs Quantum Dots

With decreasing sizes of nanostructures the problem of homogeneity becomes more and more important. Ideally, one would like to study large arrays of identical quantum systems. One elegant way to achieve this is to use fabrication mechnisms where the shape and the dimensions of the nanostructures are given by, energetic considerations, such that energy minimization will lead to the desired sample homogeneity. This happens, e.g. in the Stranski-Krastanow growth mode of InAs on GaAs heteroepitaxy and results in layers of uniform quantum dots of ~ 20 nm diameter and ~ 7 nm height. In a close collaboration with P. Petroff 's group at UC Santa Barbara we have demonstrated that these dots can be integrated into a metal-insulator-semiconductor heterostructure, which allows us to tune the electron number per dot, determine it by capacitance spectroscopy, and study the dots' excitation in the far-infrared. Here, the far-infrared spectroscopy resembles the atomic spectroscopy mentioned in the introduction. Indeed, we can "tune the dots through the table of elements" and distinguish, e.g. "quantum dot Helium" and "quantum dot Lithium" by their excitation spectrum. Furthermore, we can directly compare the different ground states of these few-electron systems, and, from a comparison between ground state and excitation energies, derive detailed information on the Coulomb and quantization contributions to the energetic structure of these man-made "atoms". A natural step further is the fabrication of "quantum dot molecules". This can be achieved because the Stranski-Krastanow islands tend to align when they are grown on top of each other. So far, no direct evidence for quantum mechanical coupling was observed, since the distance of the dots (10-20 nm) does not allow for a considerable overlap of the wave functions. However, the Coulomb interaction between the carriers in the different dot layers reveals itself in a distict shift of the many particle ground state energy. For the closely (10 nm) spaced dots, the charging sequence can be affected by an applied magnetic field.

B.5. Optics on Field-Effect Induced Tuneable Potential Superlattices in AlGaAs-GaAs Heterostructures.

In 1997, we continued our research on voltage - controlled lateral superlattices to demonstrate the trapping of photogenerated excitons. We were able to show that such a system may act as an efficient trap for neutral excitons and investigated the generation-, diffusion and relaxation processes in great detail.

Triggered by the success of the 'photon conveyor belt' described above, we also use static interdigitated gates to efficiently trap photogenerated charges in this potential landscape. Here, we can store optical information in an accumulative way in our laterally defined staorage cells. After some accumulation time, the optical storage cell can be switched to the 'storage mode' and finally be triggered externally to release the stored information in a flash of light. Even for a not optimized system, storage times in excess of 50 musec have been already demonstrated!

Presently, we are exploring possibilities to exploit these remarkable effects in terms of new, alternative and superior optical devices for detection, storage, switching, routing of optical signals as well as for optical signal processing like pattern recognition etc.

B.6. Intraband and Interband optics on self-assembled quantum dots

We have set up a system to measure the transmission coefficient of a sample at wavelengths around 1 micron with extremely low noise for interband experiments. The idea is to measure interband absorption through changes in the intensity of transmitted light. We have applied the technique to samples containing self-assembled quantum dots where the change in absorption is only about 1 part in 104 at resonance. Nevertheless, by using charge-tunable dots provided by P. Petroff 's group at UC Santa Barbara we have detected the transitions with good signal:noise. The charge-tunable dots have the advantage that we can load the dots with a discrete number of electrons and therefore measure the optical properties of charged excitons. The results show how the various interband transitions disappear according to the occupation of the dots (Pauli-blocking). Furthermore, there are energy shifts in the higher transitions as we occupy the electron ground-state. We are presently extending the work also to samples with two dot layers, and to dots provided by Harri Lipsanen, Helsinki which are defined with stressors. group

C: Nanometer Fabrication and Characterization

Figure 7. SEM micrograph of a double junction SQUID: The junctions are generated in the evaporated Al at the optically predefined constrictions. The lower right inset shows a magnification of one of the junctions -- it is clearly seen that all the material is removed. The upper inset shows a schematic representation of the nano-plough: the EBD tip isdeposited with an angle on a common AFM-tip, causing a vertical force when dragged through material.

C.1. Fabrication and characterization of semiconductor nanostructures by atomic force microscopy

Non-destructive characterization of semiconductor nanostructures by atomic force microscopy (AFM) offers the possibility to get a deeper insight into the interaction of sample fabrication and the measurement of physical properties of these structures. Furthermore, the AFM is used for nanometer-scale lithography at ambient conditions by mechanically modifying a thin resist layer. Hereby the smallest feature size is determined by the tip radius. Best results are obtained with electron beam deposited tips, which are additionally sharpened in an oxygen plasma. At the moment we are able to write holes in photoresist with a period of 9 nm and a minimal structure size of 2 to 3 nm. Meanwhile we are able to directly pattern semiconductor layers by modified EBD-tips. On the other side, a special method of local anodic oxidation has been successfully developed. Here we are able to modify metals and semiconductors with a typical smallest structure size in the range of 10 nm. Current investigations aim at the characterization and optimization of the electrical properties of such oxides for possible device application.

C.2. X-ray investigations of laterally structured surfaces

For a collaboration with M. Tolan and W. Press at Kiel University we fabricate surface gratings on silicon (period: 500 nm to 1000 nm; height: 1 nm to 50 nm) using holographic lithography followed by a dry etching process. Subsequently such gratings are covered with a thin deposit of different materials and studied by small-angle x-ray diffraction from this well defined surface roughness. For the case of block copolymers a modulation of the film surface with respect to that of the underlying substrate was found to be either in phase (conformal) or - surprisingly - out of phase (anticonformal). The critical height of the surface grating which is defined by the transition from conformal behavior to the anticonformal conformation of the films was found to be proportional to the lamellar heiht of the diblock copolymers. This has been quantitatively understood within a mean field theory as a result of balancing the deformation and interfacial energies. Furthermore, diblock copolymer films propagate the substrate roughness by significantly larger distances than obserbed for amorphous homopolymers.

In collaboration with K. Haj-Yahya, T.H. Metzger und J. Peisl at the University of Munich surface gratings on Silicon were characterized by x-ray scattering and atomic force microscopy. These periodic lateral gratings are created during rapid thermal melting and resolidification of As-implanted Si(100) substrates by laser irradiation. Structure and perfection of the grating was investigated by specular and diffuse x-ray scattering under grazing incidence and exit angles. The periodicity of the grating extracted from these measurements was confirmed by AFM studies.

C.3. Nanoscale 'Supertips' for AFM

The quality of atomic force microscope (AFM) micrographs crucially depends on the quality of the used tips. Under financial support of the Volkswagen-Stiftung and Neue Werkstoffe we have developed so called electron beam deposited (EBD) tips, which additionally can be sharpened in a special plasma etch process. This needle shaped tips are very sharp, extremely stable and chemically inert. We use this tips e.g. for high resolution topographic investigations of very deep, narrow trenches as well as for high resolution nanolithography. The feasability of these needle shaped, custom designed tips in biology, biophysics or colloid physics have been proved in a number of collaborations as e.g. with the Deutsches Krebsforschungszentrum or the Max-Planck Institut für Kolloid- und Grenzflächenforschung. As a spin-off of these activities a professional company was founded (NanoTOOLS GmbH). The tips now are commercially available and are distributed by Digital Instruments. New developments as conductive EBD tips or test samples for AFM purpose are actually in development.

C.4. The Nano-plough

Ploughing is a well-known technique since the earliest days of agricultural cultivation. To our knowledge ploughs are depicted for the first time on small clay plaques found at Uruk IV in Iraq, dated around 3200 BC.

By scaling this tool down in size to only some nanometers and combining it with conventional scanning probe techniques, we facilitate ploughing of thin film superconductors on semiconductor samples with nanometer resolution. Our technique utilizes the principle of ploughing in the same way as the traditional tool: material is removed rom the substrate in a well-defined way, leaving behind deep trenches with the characteristic shape of the plough used. The advantages of applying a nano-plough for lithography are the precision of alignment, the non-damaging definition process compared to electron or ion beam structuring techniques and the absence of additional processing steps, such as etching the substrate.

Figure 8. IV-characteristics of a weak link fabricated by employing nano-ploughing techniques.

C.5. Relaxation and Formation Dynamics of Wannier Excitons Studied by Spatiotemporal Pump and Probe Experiments

In collaboration with the group of Prof. Jochen Feldmann (Sektion Physik, University of Munich) we fabricate e-beam-lithographically defined gold structures on semiconductor quantum wells for spatial calibration purposes of femtosecond optical pump and probe experiments. The spatial resolution is in the order of 1 micron. Such spatiotemporal nonlinear optical experiments give important insight into the relaxation processes of 1s-excitons with non-vanishing in-plane center-of-mass momentum. We obtain picosecond transients of the ambipolar diffusion 'constant', which allow us to determine the temporal dynamics of formation, cooling and heating of 1s-excitons by varying the excess energy of the optically excited electron-hole pairs and the crystal temperature. We find an exciton formation time of 3 ps, exciton cooling due to acoustic phonon emission, and temperature dependent exciton heating rates governed by optical phonon scattering for temperatures higher than 50 K.

Guest Scientists, Visitors, and their Sponsors in 1997

  • Prof. Dr. Valeri Dolgopolov (Institute of Solid State Physics, Chernogolovka, Russia)
    Volkswagen Stiftung
  • Dr. A.O. Govorov (Novosibirsk, Russia)
    Humboldt Research Fellow and FOROPTO
  • Dr. Serge Huant (Grenoble, France)
    Humboldt Research Fellow
  • Dr. Aimin Song (Beijing, China)
    Humboldt Research Fellow
  • Prof. Dr. M. Shayegan (Princeton, USA)
    Humboldt Research Awardee
  • Prof. Dr. Sergio Ulloa (Ohio, USA)
    Humboldt Research Fellow
  • Dr. David Abush-Magder (Boston, USA)
    Humboldt Research Fellow
  • Dr. Håkan Pettersson (Lund, Sweden)
    TMR European Project

Diploma Theses completed in 1997

  • Stephan Wimmer
    "Magnetotransport in Anti-Dot-Übergittern mit gebrochener Symmetrie"
  • Karl Weilhammer
    "Intersubbandresonanzen an InAs-Quantentöpfen"
  • Florian Bickel
    "Entwicklung und Herstellung hochauflösender Sonden für die nahfeldoptische Mikroskopie"
  • Peter Kirchhofer
    "Untersuchung störstellenbedingter Eigenschaften des Leitwerts in ballistischen Quantenpunktkontakten"
  • Stefan Marchfelder
    "Nichtlineare Eigenschaften von Quantenpunkten im Coulombblockadebreich"
  • Heiko Seeberger
    "Herstellung und Charakterisierung der elektronischen Transporteigenschaften von SiGeC-Heterostrukturen"
  • Christoph Dürr
    "Interbandabsorption von Quantendots"
  • Lorenz Xaver
    "Aufbau und Anwendung eines Solid-Immersion-Mikroskops"
  • Artur Erbe
    "Statistische Untersuchungen von Quanten-Dot-Strukturen"
  • Markus Vogel
    "DC und AC Transporteigenschaften von Quantenpunktkontakten"
  • Stefan Böhm
    "Elektronische Eigenschaften zweidimensionaler Elektronengase in epitaktisch abgelösten Heterostrukturen"

Doctoral Theses completed in 1997

  • Martin Hartung
    "Photoleitungsspektroskopie an InAs/AlSb-Quantentöpfen"
  • Carsten Henning Rocke
    "Dynamische Modulation der Lumineszenz von Quantentopfstrukturen durch akustische Oberflächenwellen"

Publications in 1997

A. Transport Properties

  • 2-97
    D. A. Wharam
    "Transport Properties of Coupled Quantum Dot Systems"
    in "Quantum Devices and Circuits", Eds. K. Ismail, S. Bandyopadhyay, and J. P. Leburton
    (World Scientific, Singapore 1997) pp. 86-91.
  • 4-97
    D. Schmerek, S. Manus, A. O. Govorov, W. Hansen, and J. P. Kotthaus
    "Capacitance Spectroscopy of Compressible and Incompressible Stripes in a Narrow Electron Channel"
    Superlattices and Microstructures 21, 131-135 (1997).
  • 6-97
    R. Schuster, K. Ensslin, J. P. Kotthaus, G. Böhm, and W. Klein
    "Classical and Quantum Transport in Rectangular Antidot Superlattices"
    Phys. Rev. B 55, 2237-2241 (1997).
  • 7-97
    K. Ensslin and T. Schlösser
    "Quantum Transport in Lateral Superlattices"
    Physica Scripta T66, 135-137 (1997).
  • 8-97
    M. Rotter, C: Rocke, S. Böhm, A. Lorke, A. Wixforth, W. Ruile, and L. Korte
    "Single-Chip Fused Hybrids for Acousto-Electric and Acousto-Optic Applications"
    Appl. Phys. Lett. 70, 2097-2099 (1997).
  • 9-97 (postscript at LANL):
    F. Simmel, T. Heinzel, and D. A. Wharam
    "Statistics of Conductance Oscillations of a Quantum Dot in the Coulomb-Blockade Regime"
    Europhys. Lett. 38, 123-128 (1997)
  • 10-97
    A. Tilke, C. Rocke, and A. Wixforth
    "Frequency-Dependent Acousto-Electric Interaction in Proximity-Coupled Piezoelectric-Semiconductor Hybrids"
    Solid State Commun.102, 669-672 (1997).
  • 11-97
    V. T. Dolgopolov, A. A. Shashkin, M. Wendel, J. P. Kotthaus, L. W. Molenkamp, C. T. Foxon
    "Localized Electrons in the Metallic Phase of a 2D Electron System in AlGaAs/GaAs Heterojunctions"
    Phys. Rev. B 55, Rapid Comm., 7339-7342 (1997).
  • 15-97
    I. V. Zozoulenko, R. Schuster, K. -F. Berggren, and K. Ensslin
    "Ballistic Electrons in an Open Square Geometry: Selective Probing of Resonant-Energy States"
    Phys. Rev. B 55, 10209-10212 (1997).
  • 16-97
    A. G. C. Haubrich, D. A. Wharam, H. Kriegelstein, S. Manus, A. Lorke, and J. P. Kotthaus
    "Parallel Quantum-Point-Contacts as High-Frequency-Mixers"
    Appl. Phys. Lett.70, 3251-3253 (1997).
  • 18-97
    S. Lüthi, T. Vancura, K. Ensslin, R. Schuster, G. Böhm, and W. Klein
    "Electron Trajectories in Rectangular Antidot Superlattices"
    Phys. Rev. B 55, 13088-13092 (1997).
  • 19-97
    V. T. Dolgopolov, A. A. Shashkin, A. V. Aristov, D. Schmerek, W. Hansen, J. P. Kotthaus, and M. Holland
    "Direct Measurements of the Spin Gap in the Two-Dimensional Electron Gas of AlGaAs-GaAs Heterojunctions"
    Phys. Rev. Lett. 79, 729-732 (1997).
  • 25-97
    B. T. Miller, W. Hansen, S. Manus, R. J. Luyken, A. Lorke, J. P. Kotthaus, S. Huant, G. Medeiros-Ribeiro, and P. M. Petroff
    "Few-Electron Ground States of Charge-Tunable Self-Assembled Quantum Dots"
    Phys. Rev. B. 56, 6764-6769 (1997)
  • 32-97
    M. Rotter, A. Wixforth, J. P. Kotthaus, W. Ruile, D. Bernklau, and Henning Riechert
    "Quasi-Monolithic GaAs/LiNbO3-Hybrids for Acoustoelectric Applications"
    Proc. IEEE Ultrasonics Symposium, Toronto, Canada 1997, Eds. S. C. Schneider, M. Levy, B. R. McAvoy, Vol. 1, pp 201-204.

B. Intra- and Interband Spectroscopy

  • 3-97
    W. R. Frank, A. O . Govorov, J. P. Kotthaus, C. Steinebach, V. Gudmundsson, W. Hansen, and M. Holland
    "Magnetoplasmon Mode in Connected Quantum-Wire Pairs"
    Phys. Rev. B 55, 1950-1953 (1997).
  • 5-97
    L. Wendler, T. Kraft, M. Hartung. A. Berger, A. Wixforth, M. Sundaram, J. J. English, and A. C. Gossard
    "Optical Response of Grating-Coupler-Induced Intersubband Resonances: The Role of Wood's Anomalies"
    Phys. Rev. B 55, 2303-2314 (1997).
  • 12-97
    A. Lorke, M. Fricke, B. T. Miller, M. Haslinger, J. P. Kotthaus, G. Medeiros-Ribeiro, and P. M. Petroff
    "Far-Infrared and Capacitance Spectroscopy of Self-Assembled InAs Quantum Dots"
    Inst. Phys. Conf. Ser. No 155 (IOP Publishing, Bristol 1997) pp.803-808.
  • 13-97
    C. Rocke, A. Wixforth, J. P. Kotthaus, W. Klein, H. Böhm, and G. Weimann
    "Acousto-Optic Effects of Surface Acoustic Waves in Semiconductor Quantum Well Strucutres"
    Inst. Phys. Conf. Ser. No 155 (IOP Publishing, Bristol 1997) pp. 125-128.
  • 14-97
    C. Rocke, S. Zimmermann, A. Wixforth, J. P. Kotthaus, G. Böhm, and G. Weimann
    "Acoustically Driven Storage of Light in a Quantum Well"
    Phys. Rev. Lett.78, 4099-4102 (1997).
  • 17-97
    A. O. Govorov
    "Resonant Light Scattering Induced by Coulomb Interaction in Semiconductor Microstructures"
    J. Phys.: Condensed Matter 9, 4681-4690 (1997).
  • 20-97
    W. Frank, W. Hansen, A. Govorov, J. P. Kotthaus, and M. Holland
    "New Plasmon Mode in Coupled Quantum Wires in High Magnetic Fields"
    in "High Magnetic Fields in the Physics of Semiconductors II", Eds. G. Landwehr and W. Ossau
    (World Scientific Publishing, Singapore 1997), pp. 677-680.
  • 22-97
    N. J. Traynor, R. J. Warburton, M. J. Snelling, and R. T. Harley
    "Highly Nonlinear Zeeman Splitting of Excitons in Semiconductor Quantum Wells"
    Phys. Rev. B 55, 15701-15705 (1997).
  • 24-97
    P. A. Knipp, T. L. Reinecke, A. Lorke, M. Fricke, and P. M. Petroff
    "Coupling between LO Phonons and Electronic Excitations of Quantum Dots"
    Phys. Rev. B 56, 1516-1519 (1997).
  • 26-97
    A. O. Govorov
    "Inelastic Light Scattering by Electron Excitations with Large Wave Vectors in a 2D Magnetoplasma"
    JETP 85, 565-572 (1997)
  • 27-97
    P. M. Petroff, K. H. Schmidt, G. Medeiros-Ribeiro, A. Lorke, and J. P. Kotthaus
    "Size Quantization and Zero Dimensional Effects in Self-Assembled Semiconductor Quantum Dots"
    Jpn. J. Appl. Phys. 36, 4068-4072 (1997).
  • 28-97
    C. Rocke, A. O. Govorov, A. Wixforth, J. P. Kotthaus, G. Böhm, and G. Weimann
    "A Photon Assembly Line on a Semiconductor Quantum Well"
    phys. stat. sol. 164, 535-540 (1997).
  • 29-97
    E. V. Deviatov, V. T. Dolgopolov, F. I. B. Williams, B. Jager, A. Lorke, J. P. Kotthaus, and A. C. Gossard
    "Excitations of Edge Magnetoplasmons in a Two-Dimensional Electron Gas by Inductive Coupling"
    Appl. Phys. Lett. 71, 3655-3657 (1997)
  • 30-97
    R. J. Warburton, C. S. Dürr, K. Karrai, J. P. Kotthaus, G. Medeiros-Ribeiro, and P. M. Petroff
    "Charged Excitons in Self-Assembled Semiconductor Quantum Dots"
    Phys. Rev. Lett. 79, 5282-5285 (1997).
  • 31-97(postscript at LANL):
    S. Zimmermann, A. O. Govorov, W. Hansen, J. P. Kotthaus, M. Bichler, and W. Wegscheider
    "Lateral Superlattices as Voltage-Controlled Traps for Excitons"
    Phys. Rev. B. 56, 13414-13421 (1997).

C: Nanometer Fabrication and Characterization

  • 1-97
    T. H. Metzger, K. Haj-Yahya, J. Peisl, M. Wendel, H. Lorenz, J. P. Kotthaus, and G. S. Cargil III
    "Nanometer Surface Gratings on Si(100) Characterized by X-Ray Scattering under Grazing Incidence and Atomic Force Microscope"
    J. of Appl. Phys. 81, 1212-1216 (1997).
  • 21-97
    B. Irmer, M. Kehrle, H. Lorenz, and J. P. Kotthaus
    "Fabrication of Ti/TiOx Tunneling Barriers by TappingMode Atomic Force Microscopy Induced Local Oxidation"
    Appl. Phys. Lett. 71, 1733-1735 (1997).
  • 23-97
    J. M. Garcia, G. Medeiros-Ribeiro, K. Schmidt, T. Ngo, J. L. Feng, A. Lorke, J. P. Kotthaus, and P. M. Petroff
    "Intermixing and Shape Changes during the Formation of InAs Self-Assembled Quantum Dots"
    Appl. Phys. Lett. 71, 2014-2016 (1997).
  • 33-97
    Z. Li, S. Qu, M. H. Rafailovich, J. Sokolov, M. Tolan, M. S. Turner, J. Wang, S. A. Schwarz, H. Lorenz, and J. P. Kotthaus
    "Confinement of Block Copolymers on Patterned Surfaces"
    Macromolecules 30, 8410-8419 (1997).
  • 34-97
    A. Haugeneder, T. Eisenhammer, A. Mahr, J. Schneider, M. Wendel
    "Oxidation of Quasicrystalline and Crystalline AlCuFe thin films in Air"
    Thin Solid Films 307, 120-125 (1997)

D: Preprints

  • p-12/97
    R. Arnold, S. Grosse, G. von Plessen, J. Feldmann, A. Kriele, J. P. Kotthaus, R. Rettig, T. Marschner, and W. Stolz
    "Relaxation and Formation Dynamics of Wannier Excitons Studied by Spatiotemporal Pump and Probe Experiments"
    eingereicht bei Phys. Rev. B.
  • p-14/97
    S. Zimmermann, A. O. Govorov, A. Wixforth, C. Rocke, W. Hansen, J. P. Kotthaus, M. Bichler, and W. Wegscheider
    "Voltage-Controlled Trapping of Excitons and Storage of Light in Lateral Superlattices"
    Proc. MSS-8, Santa Barbara, USA, 1997.
  • p-16/97
    A. Wixforth, J. Scriba, A. Simon, C. R. Bolognesi, B. Brar, and H. Kroemer
    "Density Dependence of the Spin-Splitting of the Cyclotron Resonance in InAs/AlSb"
    Proc. NGS-8, Shanghai, China, 1997.
  • p-17/97
    A. Wixforth, C. Rocke, S. Zimmermann, J. P. Kotthaus, G. Böhm, and G. Weimann
    "Ultrasonic Light Storage in a Quantum Well: A Photon Assembly Line"
    Proc. MSS-8, Santa Barbara, USA, 1997.
  • p-18/97
    H.Kriegelstein, D. A.Wharam, J. P. Kotthaus, G. Böhm, W. Klein, G. Tränkle, and G. Weimann
    "Photoresponse of an Asymmetric Ballistic Quantum Point-Contact"
    eingereicht bei Semiconductor Science and Technology, 1997.
  • p-19/97
    R. J. Luyken, A. Lorke, M. Haslinger, B. T. Miller, M. Fricke, J. P. Kotthaus, G. Medeiros-Ribeiro, and J. M. Petroff
    "Electronic Coupling Effects in Self-Assembled InAs Quantum Dots"
    Proc. MSS-8, Santa Barbara, USA, 1997.
  • p-20/97
    M. Hartung, A. Wixforth, J. P. Kotthaus, M. Thomas, B. Brar, and H. Kroemer
    "Photoconductivity Spectroscopy of InAs/AlSb Quantum Wells"
    Proc. NGS-8, Shanghai, China, 1997.
  • p-21/97
    C. S. Dürr, R. J. Warburton, K. Karrai, J. P. Kotthaus, G. Medeiros-Ribeiro, and P. M. Petroff
    "Interband Absorption on Self-Assembled InAs Quantum Dots"
    Proc. MSS-8, Santa Barbara, USA, 1997.
  • p-22/97
    B. T. Miller, W. Hansen, S. Manus, R. J. Luyken, A. Lorke, J. P. Kotthaus, G. Medeiros-Ribeiro, and P. M. Petroff
    "Fine Structure in the Spectrum of the Few-Electron Ground States of Self-Assembled Quantum Dots"
    Proc. EP2DS, Tokyo 1997.
  • p-26/97
    V. T. Dolgopolov, G. E. Tsydnyzhapov, A. A. Shashkin, F. Hastreiter, M. Hartung, A. Wixforth, K. L. Campman, A. C. Gossard
    "Collective Gaps at Integer Total Filling Factors in a Coupled Double Quantum Well"
    eingereicht bei JETP
  • p-28/97
    M. Rotter, W. Ruile, A. Wixforth, and J. P. Kotthaus
    "Voltage Controlled SAW Velocity in GaAs/LiNbO3-Hybrids"
    eingereicht bei IEEE.
  • p-30/97
    B. Irmer, M. Kehrle, H. Lorenz, and J. P. Kotthaus
    "Nanolithography by Non-Contact AFM Induced Local Oxidation: Fabrication of Tunneling Barriers Suitable for Single Electron Devices."
    Semicond. Sci. Technol. in Druck.
  • Invited Talks in 1997

    • D. A. Wharam
      "Electronic Interactions in Coupled Quantum Dots"
      March Meeting of the American Physical Society ( Kansa City, USA, 1997)
    • J. P. Kotthaus
      "Electronic Interactions in Quantum Dots and Quantum Dot Arrays"
      Minerva Workshop on Mesoscopics, Fractals, and Neural Networks (Eilat, Israel, 1997)
    • J. P. Kotthaus
      "Manipulating Electrons in Nanostructures"
      Symposium on "Molecular Nanostructures and Low Dimensional Systems at Interfaces" (Göttingen 1997)
    • A. Wixforth
      "Ultrasonic light storage in a quantum well: a photon assembly line"
      Int. Conf. "Modulated Semiconductor Structureses", MSS (Santa Barbara 1997)
    • J. P. Kotthaus
      " Electronic Interactions in Quantum Dots"
      Int. Workshop " Novel Physics in Low-Dimensional Physics" (Dresden 1997)
    • A. Lorke
      "Far Infrared and Capacitance Spectroscopy of self-assembled InAs Quantum Dots"
      "Electronic States and Interactions in Semiconductor Quantum Dots"
      (Research Workshop in Condensed Matter Physics, Trieste 1997)
    • Richard Warburton
      "Interband Absorption on Self-Assembled InAs Quantum Dots"
      Workshop on Quantum Materials (Hamburg 1997)
    • Richard Warburton
      "Charged Excitons in Semiconductor Quantum Dots"
      Condensed Matter and Materials Physics (Exeter, UK, 1997)

    Financial Support

    Funding of this work via the following agencies is gratefully acknowledged: